[BUGFIX] Fix saving command line scheduler tasks in PHP 8

Prevent undefined array key access in saveAdditionalFields()
method in ExecuteSchedulableCommandAdditionalFieldProvider.

When the command line task is initially saved the arguments
array is not submitted.

Resolves: #95939
Releases: master
Change-Id: Iee2997330a7847659914d9083a228bec87961cae
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72136


Tested-by: core-ci <typo3@b13.com>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>